The present invention relates to the art of earth boring and, more particularly, to borehole depth measurement and measurement of drilling parameters for an oil well drilling rig. During the drilling of an oil or gas well or the like, it is necessary to obtain information on drilling parameters such as the rate of penetration through the earth formations, the depth of the well, etc. Difficulties that must be overcome in providing a system for measuring such parameters are the extremes of the working environment at a drilling rig, severe acoustical problems at a drilling rig, and electrical and radio frequency noise and other problems created by the operation of heavy machinery at the drill site. Generally, the drilling rig and equipment mounted therein are subject to severe vibration and the nature of the equipment and the workmen handling such equipment results in the equipment being subjected to severe abuse during operation.